For reasons of an excellent balance of impact strength, flow and chemical resistance a wide variety of commercial rubber-modified blends are based on styrene-acrylonitrile (SAN) copolymers. The widest commercial utility of such products is found when the rubber impact modifier phase is polybutadiene (PBD) to create the family of resins known as ABS. in order to improve the retention of impact strength and appearance upon outdoor exposure, styrene-acrylonitrile compositions comprising at least one alkyl acrylate, such as poly(butyl acrylate) (PBA) rubbers, are prepared, known as ASA (acrylonitrile-styrene-acrylate).
However, the styrene-acrylonitrile matrix polymers are significantly less stable to conditions of outdoor exposure than the PBA rubber substrate, since the styrenic structural units are more prone to photo-oxidation. Thus, systems based on styrene-acrylonitrile including ASA tend to show a tendency over time towards yellowing and chalking of the surface when exposed to actual or simulated outdoor exposure. It is well known in the art that hindered amine light stabilizers (HALS) may be added to resinous compositions in an attempt to retard the undesirable photochemistry. However, at some point the HALS is consumed at the surface of the article and yellowing can then ensue with further outdoor exposure. Thus, even ASA systems based on the more stable PBA rubber and containing HALS still show some degree of color shift and gloss loss during outdoor exposure.
By contrast, the class of impact-modified blends based on poly(methyl methacrylate) (PMMA) as the continuous rigid phase and an impact modifier based on a weatherable PBA rubber are well-recognized for showing minimal shift in color on exposure to real or simulated outdoor aging and also excellent retention of surface gloss under the same conditions. However, these blends are also often characterized by relatively low impact strength and stiff flow. A problem to be solved is to prepare compositions having the impact strength and other beneficial properties associated with compositions comprising styrene-acrylonitrile matrix polymers while obtaining the improved weatherability properties associated with compositions comprising PMMA.
Japanese patent 52-33656 to Mitsubishi Rayon discloses compositions wherein PMMA is grafted to impact modifier phase and is also a component of the rigid phase. However, these compositions do not display an optimum combination of impact, weatherability and gloss retention.